Phase-change materials, such as chalcogenides, can be switched between two states, poly-crystalline and amorphous, based on heat produced by the passage of an electric current. In the poly-crystalline state, each grain of the material is a perfect crystal and the material is conductive (almost metallic). It is notable however that each of the grains is randomly oriented with respect to the other grains resulting in an overall poly-crystalline material. In the amorphous state, there is no order in the material and the material is highly resistive. These two states make phase-change materials particularly well-suited for storing data.
To change the phase change material from the amorphous to the poly-crystalline state the material is heated above its crystallization temperature for a sufficiently long time. It will arrange itself into a poly-crystalline state during that heating time. To change the material from the poly-crystalline state to the amorphous state it will be heated above its melting temperature and then quenched (quickly cooled). It will not have time to rearrange itself in an ordered state, and an amorphous state will result. Both the crystallization temperature and melting temperature vary depending on the particular phase change material.
With conventional processes, variations in the atomic structural arrangement of the phase change material typically occur. This variation is not a great concern when large volumes of the material are used in each device. With scaling, however, the volume of the phase change material used in each device is reduced. In that case, the structure of the material can have a significant impact on device variability.
Accordingly, techniques for reducing, or eliminating, variability in a phase change material would be desirable.